Priceline's (PCLN) gross bookings still grew, but at a much more modest pace. In the second quarter of this year, bookings grew 26.8% -- the slowest growth rate since the second quarter of 2009 and far below last year's 69% surge.

Investors punished the company's stock, with shares falling 17% Wednesday. Shares of other travel sites followed suit, with Expedia (EXPE) and TripAdvisor (TRIP) both declining 5%.

Chief executive Jeffery Boyd said Priceline was hit with "economic uncertainty across Europe, Asia and the U.S. that intensified as the quarter progressed." Boyd also blamed a stronger dollar, "which put pressure on top-line growth rates."

Revenue grew just 20.3% during the quarter, compared with an increase of 43.7% in the second quarter of 2011.

The company did beat on earnings though, with earning-per-share of $7.85. Analysts had expected Priceline to earn $7.36 per share.

But investors are mainly concerned about the slowdown in bookings and the outlook for Priceline's third quarter, traditionally one of the best for travel bookings, according to analysts at Nomura Securities.

In the current quarter, the company expects international bookings to grow 23% to 31%. Analysts were expecting a 35% increase.

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The outlook is based on the assumption that "economic conditions in Europe will further deteriorate," Priceline said in its release. "The Company believes that concerns related to sovereign debt and the viability of the Euro have negatively impacted historical operating results and are likely to impact future results."

While it will probably continue to grow profits in Asian and Latina American markets, Priceline will struggle to churn out the better-than-expected profits it has reported in the past given the problems in Europe, said Dan Su, an analyst at Morningstar.

"It would take a more stabilized European economy and improving consumer sentiment in the region to return Priceline to the growth trajectory it was on in the past few years," said Su.

Ben Rooney is a staff writer for CNNMoney. He covers the European debt crisis and other international finance stories, in addition to writing about stocks, bonds, investing and other Wall Street-related news. Follow Ben on Twitter: @ben_rooney